The present disclosure relates to an image sensor and a method for manufacturing the same.
An image sensor is a semiconductor device for converting an optical image into an electric signal. The image sensor may be roughly classified into a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
During the fabrication of image sensors, a photodiode may be formed in a substrate using ion implantation. As the size of a photodiode is reduced for the purpose of increasing the number of pixels without increasing chip size, the area of a light receiving portion is also reduced, thereby resulting in a reduction in image quality.
Also, since a stack height does not reduce as much as the reduction in the area of the light receiving portion, the number o f photons incident to the light receiving portion is also reduced due to diffraction of light called Airy disk.
As an alternative to overcome this limitation, an attempt of forming a photodiode using amorphous silicon (Si), or forming a readout circuitry in a silicon (Si) substrate using a method such as wafer-to-wafer bonding, and forming a photodiode on and/or over the readout circuitry has been made (referred to as a three-dimensional (3D) image sensor). The photodiode is connected with the readout circuitry through a metal interconnection.
On the other hand, while the demand on CIS having a wide dynamic range that can simultaneously express blight light and dark light increases, products that can meet the demand are deficient. In particular, products for a 3D image sensor that can sense an infrared ray as well as a visible ray are necessary.
Since both the source and the drain at the sides of the transfer transistor are usually heavily doped with N-type impurities, a charge sharing phenomenon occurs. When the charge sharing phenomenon occurs, the sensitivity of an output image is reduced and an image error may be generated. Also, because a photo charge does not readily move between the photodiode and the readout circuitry, a dark current is generated and/or saturation and sensitivity is reduced.